1. Field of the Invention
The present invention relates to a method for detecting knocking in an internal combustion engine.
2. Brief Description of Prior Art
In a prior art apparatus, vibration of an engine or pressure in a cylinder of an engine is detected by a knock sensor such as an accelerometer, and an output signal of the knock sensor is delivered to a microcomputer which decides whether knocking occurs in the engine according to the output signal of the knock sensor. The microcomputer calculates a reference value according to the output signal through the following calculation. At first, a mean value of the absolute values of amplitudes of the output signal is calculated so as to remove noises in the signal, and the mean value is multiplied by a constant K; then an offset value is added to the result of the multiplication for compensating for deviation of performances of individual detecting systems. Thereafter, the microcomputer compares a newly detected output signal of the knock sensor with the reference value determined above and decides that the knocking occurs in the engine when the newly detected output signal exceeds the reference value. As shown by a solid line A in FIG. 1, provided that the offset value is determined to be constant, the optimum constant K has to be changed depending upon the engine speed in order to exactly detect a predetermined constant level of knocking intensity in a whole region of the engine rotational speed. In general, both an upper limit defined by a line H and a lower limit defined by a line L exist. The upper limit H is set to prevent the knock detecting apparatus from not detecting a high level of knocking and the lower limit L is set to prevent the apparatus from erroneously detecting knocking. In the conventional method, the constant K is determined to be constant, for example 4, in an area R defined by two horizontal two-dots-dash lines in FIG. 1, independently from the engine speed. The area defined by the lines H and L has substantially a U-shape, and there is a region of the engine rotational speed where the constant K in the area R is usually greater than the optimum constant K on the line A. Hence, only knocking with relatively strong intensity can be detected within the above-mentioned region, particularly at an engine speed of about 2,000 rpm where the optimum constant K is substantially minimum. This is one disadvantage of the conventional method. Another disadvantage of the conventional method is that the constant K must be precisely preset in order to keep the constant K in the area R.